The present invention is directed to optical components for use in fiber optic networks and particularly to devices known as optical isolators, and more particularly to polarization maintaining optical isolators and single polarization optical isolators.
Fiber optic communication systems rely on the use of coherent light from optical sources such as semiconductor lasers to carry optical signals. High quality transmission require sources that are stable with low noise. As the optical wave leaves the optical source and travels through the fiber optic transmission system, it encounters discontinuities at various points causing reflections. If such reflections are fed back into the source (laser), they cause instabilities which are detrimental to the performance of the source (laser) and the transmission system. Optical isolators provide a solution to this problem by allowing unobstructed flow of optical power in the forward or transmission direction while blocking all backward reflections or transmission. Therefore, by placing an isolator after the optical source, the source (laser) will be protected from backward reflections.
A commonly used isolator in fiber optic transmission systems is of the so called single polarization (SP) type. Isolation in single polarization optical isolators is achieved through manipulation of the polarization angle of the light wave. In its simplest form, a SP isolator consists of a non-reciprocal magneto-optical polarization rotation component (commonly known as a Faraday rotator), placed between two linear light polarizers, aligned such that their principal axis are at 45xc2x0 from each other. As light enters the isolator in the forward direction, it becomes polarized along the polarization axis of the input polarizer. The Faraday rotator rotates the polarization by 45xc2x0, bringing it in parallel with the polarization axis of the output polarizer. Therefore, the input optical power flows unobstructed in the forward direction.
In the backward direction, light enters the output end of the isolator and the output polarizer filters out all polarization states except the state parallel to the output polarizer, As it travels through the Faraday rotator, it undergoes an additional rotation of 45xc2x0 resulting in a polarization state at 90xc2x0 with respect to the input polarizer. The input polarizer therefore blocks, the remaining light, in the backward direction. This product does not provide transmission in the second orthogonal polarization which is why it is referred to as a xe2x80x9csingle polarizationxe2x80x9d, i.e. the output of this device will be light having only a single linear polarization and light having a polarization orthogonal to that of the first linear polarizer will not pass through the device, it does however provide isolation for both polarization states.
In a first embodiment of this invention, a true polarization maintaining isolator is disclosed. It allows transmission and isolation along the two orthogonal states of polarization (through a polarization maintaining fiber, for example). The polarization maintaining isolator of the present invention uses birefringent wedges. In the device, a nonreciprocal 45xc2x0 Faraday rotating film is placed in between two birefringent wedges having their optical axis at 45xc2x0 from each other. Lenses are used at the input and output ends to couple light out of and into an input and output polarization maintaining (PM) fibers. The major axis of the input PM fiber is aligned parallel to the optic axis of the first (or input) birefringent wedge. The major axis of the output PM fiber is aligned parallel to the optic axis of the second (or output) birefringent wedge. Because of the alignment of the axes of the fibers to the OA""s of the birefringent wedges, the amount of power in each axis (or polarization state) is maintained when transmitting through this isolator. In the reverse direction of this device, all polarization states will be blocked.
In a second embodiment of this invention a compact single polarization isolator that does not utilize a linear polarizer is disclosed. A non-reciprocal polarization rotating element having an optical polarization rotation angle of 45xc2x0 is placed in the optical path between two birefringent wedges that are aligned with their optical axis at 45xc2x0 from each other. In this configuration, the two orthogonal states of polarization of the incoming light beam will exit the output wedge with their polarization state switched with respect to the output birefringent wedge""s optic axis and separated by an angle. A pair of lenses are used to collimate the optical beam in the polarization rotating element region of the device and couple light into and out of the input and output fibers. With a linearly polarized light input by the input fiber (which can be a polarization maintaining fiber), the output fiber (which can also be a polarization maintaining fiber) is aligned (positioned with respect to the output lens) so that only one of the two polarization states is coupled (lensed) into it.